This invention relates to a programmable gain feedback amplifier. High gain differential amplifiers, particularly those fabricated by microelectronic techniques, are quite often used in applications whereby feedback is established from the output to the input by a network, the attenuation of which sets the closed loop gain of the amplifier circuit. The ability to improve the linearity and stability of an amplifier by use of negative feedback is demonstrated and described with reference to curves or plots and vector diagrams in addition to illustrative circuit diagrams in U.S. Pat. No. 2,102,671, granted to Harold S. Black on Dec. 21, 1937.
In general, providing negative feedback in an amplifier consists of coupling a portion of its output signal E.sub.o to its input terminal, but out of phase with the input signal E.sub.i. In this way, distortions in the signal being amplified are corrected by the portion of the output signal fed back to the input terminal.
If the amplifier gain in the absence of feedback is A (the open loop gain), the output signal E.sub.o is equal to AE.sub.i. If the portion of the output signal fed back is B, the feedback signal E.sub.f is equal to .beta.E.sub.o where .beta. is the factor by which the output signal E.sub.o is multiplied to obtain E.sub.f and is a characteristic of the feedback circuit alone. Note that .beta. may be a complex quantity, depending on the feedback circuit design, and that A is usually a complex quantity.
The expression for the output signal E.sub.o with the negative feedback circuit connected to the input terminal of the amplifier is: EQU E.sub.o =A(E.sub.i +E.sub.f)
By substituting .beta.E.sub.o for E.sub.f and solving for E.sub.o, the following expression is obtained: ##EQU1##
Because the closed loop gain is by definition the ratio E.sub.o /E.sub.i, the closed loop gain is found to be: ##EQU2##
If A is very large, the closed-loop gain is approximately equal to EQU -1/.beta.
which depends only upon the value of .beta..
It is currently a common practice for a microelectronic high gain amplifier to be used in combination with an external feedback network, usually composed of discrete resistors to set the circuit gain. It has been shown, however, that the gain of a feedback amplifier can be programmed or controlled by switching in the proper resistive elements in the feedback (or input) networks by relay switches in U.S. Pat. No. 3,153,202 granted to Frank J. Woolam on Oct. 13, 1964, or by transistor switches as in U.S. Pat. No. 3,629,720 granted to Adel S. Sedra and Kenneth C. Smith on Dec. 21, 1971.
Under these circuits, the gain setting procedure is not permanent. Additionally, the gain control signal must be continuously applied. Finally, the circuit required to achieve the programmable gain is relatively complex.
Digital microelectronic devices, particularly programmable read only memories, have utilized a very simple mechanism for altering the circuit by a programming or controlling signal, namely the fusible element or fusible link.